When state of the art baseband methods are used, the range decreases with increasing transmission rate. This means that when the transmission rate is increased, the data to be transmitted can only be supplied to subscribers via trunk lines of decreasing length. If, with a predetermined distance and a selected rate, the limit is already reached, it is no longer possible to switch to an even higher rate. Since the distance to the limit is often unknown, one possibility of solving the problem is to change the rate in small steps. However, this frequently results in performance being given away. As an alternative, in principle, only those ranges can be selected at which it is ensured that they can be bridged even at the maximum possible rate.
If the transmission rate between the transmitting device and the receiving devices is to be increased, this is done such that both stations (i.e. transmitting device and receiving devices) operate at the same increased rate. For this purpose, the receiving device in question in each case must be informed by one of the stations such as, e.g. the transmitting device, that an increase in transmission rate is required and corresponding setting instructions must also be transmitted for this purpose.
The communication between transmitting device and receiving device required for this purpose takes place via a separate channel. This can be designed as an independent outband channel (user data and control data (signaling data) are conducted via different links) or inband channel (user data and control data are conducted via one link). In the latter case, care must be taken, with appropriate reliability of range, that the communication between the two stations is still ensured after switching up to a higher rate. In cases of doubt, therefore, the rate for an existing connection must be maintained.
Increasing the transmission rate above the critical limit presents problems since the level arriving at the receiving site is too low and, consequently, synchronism between the transmitting device and the receiving device is lost (“receiving device disconnecting”) and thus communication between the two stations is no longer possible. The reason for this is that in this case, the transmitting device has already adjusted itself to the increased rate but the remote station is still operating at the old rate by itself.
Communication is then no longer possible since the remote station is “disconnected”. Interworking is only possible again when both stations are manually set to the same rate.